This invention relates to the field of light-weighted optics, and more particularly to a light-weighted optical assembly and a method of constructing a light-weighted optical assembly.
Light-weighted optics is a field which has developed predominantly due to the space program, and the need to reduce weight of optical instruments and the materials used in producing those optical instruments. Since many experiments conducted in space and the functioning of satellites sent into orbit involve the need to reflect light, i.e., such as in a telescope, light-weighted optics have become old in the art.
The thickness of an optical assembly, and accordingly, its weight are directly related to the substrate material used to construct the optical assembly and the area of the reflective surface of the optical assembly. Choice of substrate material is directly related to the substrate's stiffness and to the substrate's coefficient of thermal expansion. It is the stiffness and coefficient of thermal expansion of the substrate which prevent distortion of the reflective surface due to such variables as the weight of the optical assembly and temperature change. For example, a 3'.times.3'optical assembly (such as a mirror), produced on a substrate having a stiffness ("aspect") ratio of 6 (a normal aspect ratio in the field of optics) would require approximately an 8-inch thick substrate, according to standard practices in the field. It is evident therefore that such a 3'.times.3'.times.8" optical assembly would have considerable mass.
Accordingly, it has been shown in the field that weights of optical assemblies can be reduced if the substrate material is reduced. However, since the thickness of the optical assembly with respect to the surface area of the reflective surface can not be varied, different methods of reducing the quantity of substrate material have been developed. Such methods of reducing substrate weight are seen in the following issued patents.
U.S. Pat. No. 3,507,737 to Busdiecker et al. is for an invention directed to the Art of Sealing Thermally Crystallizable Glass, and Thermally Crystallizable Telescope Mirror Blanks. As seen in FIGS. 1-6 of the '737 patent, a telescope mirror blank 9 can either be constructed of substrate layers 10 and 13 separated by substrate spacing members 11 and 12, or of substrate layers 23 and 24 separated by individual substrate spacing members 25 or 29. Prior to the configurations of the '737 patent, such a mirror would essentially have been constructed of a solid substrate mass. This solid substrate would have had a volume defined by the surface area of layer 10 and the thickness of mirror 9. [The thickness of mirror 9 of the '737 patent is defined by the thickness of layers 10 and 13 plus the height of members 11 and 12.] However, the substrate configuration of the ' 737 patent reduces the overall weight of mirror 9 because the majority of the volume occupied by the substrate of mirror blank 9, is not solid.
The substrate configuration of the '737 patent is seen to consist of interlocking spacing members 11 and 12 (FIGS. 1-3), or a plurality of individual spacing members 25 or 29 (FIGS. 4-6). However, even though these substrate configurations reduce the weight of mirror 9, a number of disadvantages stem from the methods of constructing the '737 mirrors: 1) the securing together of interlocking members 11 and 12 and the non-uniform nature of having to connect upper and lower members 10 and 13 or 23 and 24 to spacing members 11 and 12, 25 or 29, respectively, creates increased areas of possible failure and therefore a heightened probability of mirror distortion; and 2) the structures are inherently more difficult to construct.
U.S. Pat. No. 3,912,380 to Klein for a Composite Type Structure for Large Reflective Mirrors shows another previous method of constructing light-weighted mirrors. As shown in FIG. 4 of the '380 patent the mirror is predominantly constructed of a honeycomb structure 12. Attached to one surface of honeycomb structure 12 is a series of three plates; top plate 22, isolator 16 and mirror 14 having reflective surface 24. The construction of the '380 patent is prohibitive since the formation of honeycomb structure 12 is costly and the inherent nature of the 3-piece (pieces 22, 16 and 24) reflecting structure lends itself to a less accurate mirror.
Finally, U.S. Pat. No. 4,917,934 to Sempolinski for a Telescope Mirror Blank and Method of Production shows in FIGS. 1-5 a light-weighted mirror having an assembly 40 constructed of interlocking struts 32 having slots 34 adapted to receive a cross strut 36 having slots 38. This method of forming the inner core of the light-weighted mirror shown in FIGS. 1-5 of the '934 patent is similar to that of the method shown in FIGS. 1-3 of the '737 patent discussed above, and has the same inherent shortfalls. A second embodiment of the '934 patent, shown in FIGS. 6 and 7, is directed to a plurality of tubes 60 which are joined together to make a honeycombed core assembly 70. Both of the structures of the '934 patent are difficult to construct and have many areas of possible failure. Therefore, it is less likely to achieve accuracy with these mirrors, or if the desired precision is achieved, it is more likely that it will be lost due to structural failure.
Accordingly, it would be desirable to provide a light-weighted optical assembly having fewer connected pieces and no interlocking pieces, while still maintaining rigidity and stiffness so as to avoid deflection of any reflective surface, while also achieving substantial weight reduction of the overall optical assembly.